1. Field of the Invention
The present invention relates to an image forming apparatus using the electrophotographic method for, such as a photocopier, printer, facsimile machine, or a multi-function machine, and the developing device and process cartridge installed therein. More particularly the present invention relates to a premixed developing system developing device in which new carrier is supplied to the developing device as appropriate, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
Conventionally, in image forming apparatus such as multi-purpose machines, printers, and the like, technology (referred to as the premixed developing system) for supplying new carrier to a developing device that contains two part developing agent made from toner and carrier (including the case where additives or the like are added) is disclosed in, for example, Japanese Patent Application Laid-open No. 2001-183893 and Japanese Patent Application Laid-open No. 2000-112238, and elsewhere.
In a developing device using two component developing agent, as toner is consumed within the developing device, toner is replenished as appropriate into the developing device through a toner replenishing aperture provided in a part of the developing device. The supplied toner is agitated and mixed with the developing agent within the developing device by a transport member (agitation member) such as a screw or the like. A part of the agitated and mixed developing agent is supplied to a developing roller. The developing agent on the developing roller is controlled to an appropriate amount by a doctor blade. Then at the position in opposition to the photosensitive drum, the toner in the two part developing agent adheres to the latent image on the photosensitive drum.
In this way, in the normal developing process the carrier in the two part developing agent housed in the developing device is not consumed but remains within the developing device, so the carrier becomes degraded with the passage of time. In more detail, as a result of agitation and mixing of the carrier within the developing device over a long period of time, a “film depletion phenomenon” occurs, in which the coating layer of the carrier becomes worn or peels off, which reduces the carrier charging capacity, or a “spent phenomenon” occurs in which components of toner or additives adhere to the surface of the carrier, which reduces the carrier charging capacity.
The premixed developing system is a system to prevent the reduction in image quality of the images output as a result of this degradation of the carrier with time. In other words, by supplying new carrier (or new two part developing agent) as appropriate to the developing device, and discharging a part of the two part developing agent housed within the developing device outside the developing device as appropriate, the degraded carrier within the developing device is reduced, and it is possible to maintain the quantity of carrier housed within the developing device and its charging capacity.
Compared with image forming apparatus for which it is necessary to change the developing device or carrier with new product every time degradation of the carrier with time occurs, the quality of the images output from an image forming apparatus that uses the premixed developing system as described above is more stable with time.
In Japanese Patent Application Laid-open No. 2001-183893 and Japanese Patent Application Laid-open No. 2000-112238, image forming apparatus using the premixed developing system are disclosed in which an overflow system is used as discharge means for discharging the developing agent outside the developing device. In detail, a discharge aperture (hole) is provided in the wall of the developing device, and when the surface of the developing agent transported to this position exceeds a predetermined level, the developing agent (the developing agent that has become excess as a result of replenishing with carrier) is discharged to the outside from the discharge aperture.
Also, in Japanese Patent Application Laid-open No. 2000-112238, in order to reduce the quantity of developing agent discharged from the discharge aperture (developing agent discharge aperture), technology is disclosed in which the screw of the transport member (agitation and transport member) in opposition to the discharge aperture is removed (or reduced in diameter).
In the premixed developing system developing device according to Japanese Patent Application Laid-open No. 2001-183893, in addition to the developing agent that was intended to be discharged, the necessary developing agent was also discharged from the discharge aperture. As a result, the developing agent within the developing device was excessively discharged, and the developing agent became insufficient. Specifically, although only developing agent whose surface height exceeded a predetermined height was intended to be discharged, developing agent that was thrown up by the transport member was also discharged from the discharge aperture.
When the amount of developing agent within the developing device becomes insufficient in this way, the state of degradation of the developing agent becomes unstable, the amount of charge on the toner is reduced, the image density of the images output is reduced, and other problems occur.
In order to solve this problem, in Japanese Patent Application Laid-open No. 2000-112238, the screw is eliminated (or reduced in diameter) from the transport member opposite the discharge aperture. However, as a result of the researches by the inventors of the present application, it was found that by eliminating the screw from the transport member opposite the discharge aperture alone, it was not possible to sufficiently reduce the discharge from the discharge aperture of the developing agent thrown up by the transport member.